Metallic foam material

ABSTRACT

The invention provides a metallic foam material with better mechanical properties than known foamed materials, and a method of making such a material. Although known foamed metals are light in weight, and sandwich structures formed of such materials can be formed into structural components, the tendency for the structure to crush and fracture under compressive loading, with consequent crack propagation, limits their use in applications in which the integrity of the component is important. The invention addresses this problem by providing a fibre-reinforced foam that combines the tensile strength of a high strength fibre such as carbon fibres with the impact resistance (through crushing and deformation) of metallic foam. The metal foam imparts a greater strength to the carbon-fibre-reinforced plastics (CFRP) part of the structure than it would have by itself. Under tensile loads, the fibre reinforcement gives the metallic foam enhanced strength and low creep.

This invention relates to metallic foam materials. In particular the invention relates to metallic foam materials with fibre reinforcement.

Metallic foam materials have been known and used for some decades (at least since the 1960s). Several methods are known for making such materials; for example, a number are listed in the book “Metal Foams—A Design Guide” by Ashby, Fleck et al. (ISBN 0750672196).

Metallic foam materials with a wide range of cell sizes, and consequently with a wide range of relative densities (compared with solid), can be produced.

Frequently, such materials are used in applications (such as air/oil separators, electrodes for electrochemical cells, or catalytic convertors) in which the most important properties are their porosity, uniformity of resistance to fluid flow and large surface area. In such applications, the generally low mechanical strength of these materials is not detrimental.

It is possible to form structural components from metallic foam materials, but the mechanical strength of these components is generally relatively low per unit volume, though in some structures the strength/weight ratio is good.

Under compressive loads, the structure crushes and fractures, and from these fractures cracks can propagate. In some applications, this can be a useful feature, but—because the fracturing and cracking can result in parts of the structure breaking off—such materials cannot be used in applications in which the component must retain its integrity under loading.

Under tensile loads, the ligands (the filaments joining the cells) creep and elongate, which again makes the materials unsuitable for some applications. Although it may be possible to increase the tensile strength by adding material, inevitably this will increase the structure's weight.

It would be desirable to have a metallic foam material in which greater mechanical strength could be achieved, without compromising its other useful properties.

Accordingly, the invention provides a metallic foam material and a method of making a reinforced metallic foam material as set out in the claims.

Embodiments of the invention will now be described in more detail, with reference to the attached drawings, in which

FIG. 1 shows polymer beads threaded on a fibre tow;

FIG. 2 is a schematic view of a mould assembly;

FIG. 3 is a schematic view of a rotating mould assembly;

FIG. 4 is a schematic view of creating a carbonised foam precursor prior to an electroforming operation;

FIG. 5 is a schematic view of a part of a structure of beads and fibres subjected to the application of heat; and

FIG. 6 is a schematic view of beads with fibres in more than one direction.

The first step in making a fibre reinforced metallic foam material according to the first embodiment of this invention is to make a precursor. The precursor comprises a plurality of polymer beads arranged along a length of reinforcing fibre.

In a first embodiment of this step, polymeric beads are threaded on to a tow of carbon fibres. To facilitate the threading, a metallic tip is bonded to the fibre tow. This prevents the fibres shaling, spreading or breaking. In this embodiment, the tow consists of some 12000 fibres and the polymeric beads are made of carbon-coated polystyrene. The carbon coating may be applied by any suitable means, for example by spraying or dipping. FIG. 1 shows such a precursor, comprising a fibre tow 12 with polymer beads 14 threaded on it.

It was found that, with this technique, the beads became loose after a short length of fibre had been threaded, because the fibres were beginning to cut through the beads and were not all remaining in their tow. Twisting the fibre tow slightly improved the performance, but did not eliminate the cutting of the beads and increased the risk of fibre breakage.

In a second embodiment of this step, the beads are moulded around the fibre tow. FIG. 2 shows schematically a mould formed in two parts 18, 20. Recesses 22, 24 in the mould are filled with polymer (not shown) before a fibre tow 26 is placed between the mould parts 18, 20. When the mould is closed, the recesses 22, 24 cooperate to form polymer beads around the fibre tow.

In a third embodiment of this step, the beads are moulded around the fibre tow in a continuous process using a rotating mould, as shown schematically in FIG. 3. A fibre tow 30 is fed continuously through a circular mould 32. The mould rotates in the direction of the arrow at a speed to match the feed rate of the fibre tow. Polymer is fed into the mould as it rotates so that beads 34 are formed around the fibre tow 30 as it is fed through the mould, producing a continuous precursor 36.

In a fourth embodiment of this step (not shown in the drawings), a uniform coating of polymer may be applied to the fibre tow 30 before it enters the mould 32, so that the rotating mould compresses the polymer to form the beads in a continuous manner. Because this embodiment does not require a supply of polymer into the rotating mould 32, it may avoid problems with flow of polymer into the mould.

In the second to fourth embodiments of this step, the beads can be cured after moulding, by any means suitable for the polymer used—for example, by a cooling air blast for heat foamed polymer or by passing the precursor through a ring lamp for UV catalytic cured polymer, or by absorbing moisture from the air for a diisocyanate foam.

If required, a conductive coating may be applied to the beads, for example a coating of carbon or of copper paste or dust. Spraying and dipping are suitable methods for applying the conductive coating, but any other suitable method may be used instead.

The second step in making a reinforced metallic foam material according to the first embodiment of the invention is to assemble the precursor strings of beads into a processing vessel. This may require the strings to be cut to length.

Generally, the precursors will be arranged so that the beads form a hexagonal close-packed array; though they may be arranged in other patterns or in a pseudo-random fashion. This last arrangement may be advantageous because it enables better interlocking of the beads between different layers of the material, thereby improving its mechanical strength. This can for instance be done by alternating two complementary sizes of beads on the strings; by lining up the larger sizes with the smaller sizes when laying up, the beads will form an interlocking pattern.

The third step in making a reinforced metallic foam material according to the first embodiment of the invention is to deposit metal on the surfaces of the beads to form the metallic foam. This electroforming process may be performed by a known method such as electroplating (as shown, for example, in FIG. 4 of U.S. Pat. No. 3,694,325).

A first embodiment of this step is suitable for beads that have had a conductive coating applied to them in an earlier step. Because the beads have a conductive coating, they form part of the electrical plating circuit. Clearly, in order to deposit metal the electrolyte needs to flow through the structure, which with this arrangement and method of manufacture relies on the inter-bead space. The electrolyte pumping pressure is arranged to be high enough to break through the surfaces of contact of the beads, allowing the electrolyte to flow through the spaces between the beads. The flow 40 of electrolyte is sufficient to break through the surfaces of contact 42 of the beads 44, forming a continuous space through which the electrolyte can flow. As the electroplating proceeds, the size of this space reduces until the flow stops. For structures up to around 150 pores (or beads) thick this method produces a near-hollow metallic structure between the beads, which gives a very good strength-to-weight ratio. For thicker structures, the resistance to flow of the structure means that the required electrolyte pumping pressure can damage the metallic filaments or cause discontinuities.

In a second embodiment of this step, the beads are not coated with a conductive coating. Instead they are coated with an organic resin, preferably one that will carbonise at a relatively low temperature, typically below about 180° C. This carbonisation is done for several reasons—firstly, to seal the surface and form a good contact with the next bead when the beads are laid in the mould; secondly, to ensure that a conductive carbon ligament is formed from the carbonised resin; and thirdly, to support the structure whilst the bead body is collapsed (see FIG. 4) during the subsequent processing by pressurising the structure to collapse the walls to create an open foam structure.

If the fibre tow is not to be plated/metallised in this second embodiment, then it should be coated with a non-conductive and high-surface-tension material. The temperature at which this coating (or, if using a pre-preg tow, the matrix) will carbonise should be higher than that at which the beads and their organic coating will carbonise. This can be achieved, for example, by using a high temperature epoxy resin to coat the tow, such as a BMI (bismaleimide) or polyimide resin. Alternatively, a glass coating may be used on the tows (as described in UK patent application GB2467366), which will satisfy the temperature requirement and also impart improvements to the structural failure characteristics. The glass provides a higher elongation to failure and protects the fibre tow during the manufacturing process, acting also as a moisture and oxygen barrier between the fibres in the tow and the rest of the structure. For components made to net shape, the glass may be sealed over the ends of the tows under low pressure.

If the fibre tow is to be metallised, then any sizing that is used either must volatilise or it must be cleaned immediately before the process. Selective cleaning, for example with a laser, may be used to form patterns on the fibre tow. For instance, using a pre-preg tow, the inventor has used a ‘T’-class laser to remove the organic matrix (resin) in a double helix pattern around the fibre tow. During the electroforming process, the fibre tow was therefore only be plated with metal in those regions where the matrix had been removed. The result was a braid-like metallic surround (looking much like a stent) grown around the fibre tow. It is envisaged that this technique could be extended beyond the beaded portion to assist with attaching or interfacing the resulting component to other structure or other materials.

As with conventional metallic foams, foams made by this method can be made with closed cells in one or more directions. This can be achieved by the position and the way the beads are compressed in the electroplating vessel. This may be advantageous, for example, to form a barrier layer (e.g. for fire protection) or to form an acoustic cell.

In a third embodiment of this step, following the step of coating the beads with an organic resin, a temperature treatment cycle is performed on the assembled precursors to collapse the beads on to the organic resin coating. This step is shown schematically in FIG. 5, in which three fibres 48 are surrounded by three beads 50. The beads have a resin coating 54. It is important to control this stage closely, depending on whether or not the fibres are to be metallised. By using the (heat or electrical) conductivity of the fibres, heat is applied to make the beads 50 shrink outwards 52 away from the fibre tow 48 and on to the resin coating 54 of the strings before the carbonising process is started. This will cause gaps to open up between the beads and the fibre tows. As the heat causes the beads to soften, the pressure between them will cause the contact faces between them to flatten slightly, creating areas of contact between the beads as shown by the solid lines 56 in FIG. 5. It is important to control this stage carefully to ensure that the bead structure does not collapse due to the softening of the beads and the organic resin coating. As an example, where the bead is formed from polystyrene expanded with a hydrocarbon or fluorohydrocarbon and the beads have been coated with a polyepoxide (epoxy—thermoplastic polymer) that is aged and cured after assembly, with cure temperature being limited to 135° C. The polystyrene bead can be preheated using the fibres and shrinkage will start around 45° C., progressing more rapidly as the temperature increases.

As the temperature increases during the temperature treatment cycle, the (e.g. polystyrene) filaments formed from the beads start to carbonise. The flow of vent gas needs to be adjusted to control the oxygen flow—oxygen is needed to reduce the organic resin coating and styrene to carbon. It is important, though, not to oxidise the carbon, but only to provide sufficient oxygen to remove the hydrogen from the H—C bonds. If the process “overcooks”, then the carbon structure becomes very brittle and can collapse during the introduction of the electrolyte. With extreme “overcooking” some of the carbon can be oxidised, thus reducing the structure and (because the process is exothermic) risking thermal runaway. If the cycle is not completed, or the temperature is initially too high, then carbon forms a barrier on the outside of the filaments and the filaments will be fatter, but with an internal core that contributes little strength to the overall structure but adds weight.

When this technique is used to make near-net-shape components, the original design has to be larger because the beads and coating shrink (as in FIG. 5) when the temperature is raised for carbonisation. This can be modelled and accounted for in the design of the mould for the component. The amount of change depends on a number of factors, principally the materials used, the process parameters and the bead material density. It can be as much as 10%, but with careful design can be reduced to 1% or so.

This carbonisation process produces a reticulated conductive structure, which can then be subjected to an electro-forming process, as previously described, to form a metallic foam material in accordance with one aspect of the invention. Because the reticulated structure is relatively open, having potentially thinner filaments than in the first embodiment of this step, it is easier for the electrolyte to flow through it and so a lower electrolyte pressure can be used. This means that this embodiment of the third step can be used to make thicker components.

A further insight of the inventor is that in the first embodiment of the third step, described above, it is not necessary to form the beads of sacrificial material. In a particular preferred embodiment, the beads are formed from matrix material, preferably compatible with the carbon fibre and any component this will be joined to. Typically such a CFRP matrix material will be a polyimide epoxy or another thermoplastic polymer. Doing this saves time later in the manufacturing process if matrix material is to be infiltrated into the foam. This, for example, can be where a reinforced part of a composite structure is required; the fibre reinforced foam is made to net shape and then co-moulded or co-cured with the rest of the composite component to form a completed component with a reinforced section. It will be appreciated that part of the material can be formed in this way and part left open as in the earlier description, which allows a different material (or none) to be attached. For instance, this could form a composite structure that has a reinforced surface to resist impact, the surface of which is impregnated with resilient material to form resistance to small particles and provide further beneficial properties such as radar absorption. Other materials that have been used for the beads include carbonised neoprene, pre-preg fibre tow, and matrix material combined with some filler material such as chopped fibre, hollow spheres or clay. In principle the beads need not be made from a single material, but could be made from a number of layers or ‘shells’ of different materials to deliver particular combinations of properties. This has the advantage in a composite material that, for instance, around the epoxy bead a layer of thermoset plastic could be provided to act as an interlayer toughener or damper. Alternatively a bead of clay could be surrounded by resilient material to act as a particle damper.

In the particular preferred embodiment in which the beads are made from matrix material, the matrix must be part-cured (to just beyond gelation) so that when building thick structures the lower layers do not become distorted by the weight of the layers above. For a standard epoxy based matrix material the temperature was raised to around 135° C. to start the gelation, and then reduced slightly to around 120° C. A disadvantage of this technique is that the structure is relatively stiff when cooled, and so it cannot be so readily moulded to shape in subsequent processing steps.

When using an embodiment where the matrix beads have a conductive coating, it was found that the usual carbon or copper paste mixtures caused problems with consolidation and curing of the matrix where the beads interact, preventing full diffusion and bonding. A technique of putting a dry carbon coating in the mould worked, with the carbon tending to diffuse into the matrix material, and also acting as a release agent for the mould. Careful application of the conductive paste to the mould ensured good matrix bead contact. Even better results were obtained by releasing the matrix beaded tow (i.e. a string of beads moulded on a fibre tow) at a temperature of around 70 to 100° C. (i.e. before it was fully cooled) and applying another thin layer of carbon while the matrix material was still tacky. This was conveniently achieved by using a small spray gun to blow a dusting of carbon powder on to the matrix surface.

In a further embodiment of a metallic foam material in accordance with the invention, some of the fibre tows are left uncoated and are passed out through a wall of the electroplating mould/processing vessel. The formed component will then have a “hairy” edge, which can facilitate the integration of the metallic foam component into another component by providing features (the protruding tows) that can be laid or co-moulded or otherwise secured into another part of the component or assembly.

The invention thus provides a metallic foam material with better mechanical properties than known foamed materials, and a method of making such a material. Although known foamed metals are light in weight, and sandwich structures formed of such materials can be formed into structural components, the tendency for the structure to crush and fracture under compressive loading, with consequent crack propagation, limits their use in applications in which the integrity of the component is important. The invention addresses this problem by providing a fibre-reinforced foam that combines the tensile strength of a high strength fibre such as carbon fibres with the impact resistance (through crushing and deformation) of metallic foam. The metal foam imparts a greater strength to the carbon-fibre-reinforced plastics (CFRP) part of the structure than it would have by itself. Under tensile loads, the fibre reinforcement gives the metallic foam enhanced strength and low creep which it would not have on its own.

The principal advantage offered by the material is that components can be made with lighter weight. In addition, the invention enables the production of structural components with light weight, high impact resistance and good acoustic attenuation using a single manufacturing process.

Various modifications can be made without departing from the scope of the invention.

For example, the beads may be made from other materials besides those disclosed. In a particular embodiment of the invention, the beads are made of wax, which can be melted or dissolved away to leave only the organic resin coating, which will then form the reticulated structure. Ceramic beads may also be used. A combination of bead types can be used so as to allow a filler and an open cell structure, or more than one filler and/or open cell structure.

Other fibre materials may be used in place of carbon; for example, glass, metal, boron, silica, aramid (e.g. Kevlar), neoprene or a combination of different materials. An example already discussed in this specification is the use of glass-coated carbon fibres.

Some cells may be made closed-cell, or partially closed-cell, by the application during layup of a coating (such as a resin). This technique could be used, for instance, to form a barrier part-way through a component (such as a septum layer); or to divide the structure up into discrete cells, each with an open-cell structure within. This would permit large acoustic resonator cells to be created, but with an internal structure to provide strength with minimal increase in weight. One application of this would be to make low-frequency Helmholtz resonators from sets of linked cells within the structure and to have a range of surrounding cells that act as broadband acoustic dampers. Another possibility would be to form a layer, like a continuous sheet, through the structure; this could act as an impermeable barrier against gas or another fluid. It could also be used as a thermal transfer boundary, to allow heat transfer between two fluids without mixing.

The fibre reinforcement may be provided in more than one direction. Factors that will need to be considered include the fibre fill required, the close packing matrix used in the foam precursor, and whether fibre kinking (from the interaction of the fibres) is acceptable. For example, as shown in FIG. 6, fibre tows 58 are arranged at −60° and fibre tows 60 at +60°. Beads 62 are formed at the positions where the fibres intersect. It may be convenient to use a braided mat to provide the required disposition of fibres, though (as with any woven structure) there will be local reductions in the material properties caused by kinking of the fibres at the weave joints. Unidirectional fibres will yield a stronger result and, by using an offset lay-up pattern, two sets of unidirectional fibres at the required angles can be laid up within the metal foam as shown in FIG. 6.

Chopped fibre may be moulded into the precursor; this can either remain in the final product or can be replaced by a filler material. This has the advantage of increasing the fibre volume fraction of the component and is useful where the metal foam is effectively forming a matrix through a CFRP structure or component. As the resin and chopped fibre are infused into the complete foam and cured under pressure they form a bond around the metallic structure. Surprisingly, the CFRP acts as a crack stop for the metallic filaments, whereas the converse would be expected.

The invention provides a metallic foam material which is suitable for applications in which a measure of tensile strength is required as well as crush, impact or abrasion resistance. This may be in applications analogous to those already described in relation to gas turbine engines, such as containment structures, casings or surfaces with integrated acoustic tiling. Other possible applications for the invention would be in applications such as crash barriers for roads or protection panels for satellites that form part of the structure to reduce parasitic weight, where current production methods make such materials too expensive to use. The metallic foam material of the invention, and lightweight hybrid structures formed from it, can be homogeneous, multilayered, formed as a sandwich structure, or moulded. 

1-18. (canceled)
 19. A metallic foam material in which at least part of the foam is reinforced by fibres embedded within it.
 20. A material as claimed in claim 19, in which the foam comprises cells.
 21. A material as claimed in claim 20, in which the cells are of uniform size and are arranged in a hexagonal close-packed array.
 22. A material as claimed in claim 20, in which the cells are not of uniform size and are arranged in a pseudo-random manner.
 23. A material as claimed in claim 19, in which the fibres are carbon.
 24. A method of making a reinforced metallic foam material, comprising the steps of: a) constructing a precursor comprising a plurality of beads secured to a fibre; b) arranging a plurality of the precursors in a processing vessel; c) modifying the beads to form a cellular structure reinforced by the fibres.
 25. A method as claimed in claim 24, in which step a) comprises forming the beads by moulding them around the fibre.
 26. A method as claimed in claim 25, in which the moulding step comprises coating the fibre with the bead material and then actuating the mould to form the beads around the fibre.
 27. A method as claimed in claim 24, in which step a) comprises curing the beads.
 28. A method as claimed in claim 24, in which step a) comprises coating the beads in carbon.
 29. A method as claimed in claim 24, in which step b) comprises arranging the precursors so that the beads form a hexagonal close-packed array.
 30. A method as claimed in claim 24, which comprises before step c) the step of arranging in the processing vessel additional beads that are not secured to a fibre.
 31. A method as claimed in claim 30, in which the additional beads form in step c) a cellular structure that is integral with or attached to the cellular structure formed by the precursors.
 32. A method as claimed in claim 24, in which step c) comprises an electroforming process.
 33. A method as claimed in claim 32, in which the electroforming process is preceded by a heating process to carbonise the beads. 